This invention relates to a process and apparatus for ion exchange by use of a thermally regenerable resin of a heterogeneous type, and more particularly to continuous or semi-continuous ion exchange based on a movable bed of thermally regenerable resin in a single column.
In ion exchange treatments using ion exchange resins of the type which are regenerated with chemical reagents, when the treatment is performed in a fixed bed-type ion exchange system, the ion exchange treatment and the regeneration treatment are not performed simultaneously within one and the same column. Rather, one of the two treatments is shut down while the other treatment is in progress. In an improvement on such system, a continuous moving bed-type ion exchange system has been developed and is in practical use. However, the improved system nevertheless has the disadvantage that the cost of equipment is high because a separate regeneration column must be installed in addition to the ion exchange column.
Thermally regenerable resins, such as those used in the present invention, are resins which, unlike any of the conventional ion exchange resins capable of being regenerated in ion exchange capacity by the use of chemical reagents (such as aqueous solutions of acids and alkalis), can have their ion exchange capacity regenerated by hot water alone. Such resins are now commercially available, one example being "Amberlite" (registered trademark) XD-2, a product of Rohm and Haas Company, United States.
Accordingly, an object of this invention is to provide a process and apparatus requiring only a single column, wherein the column is packed with a heterogeneous thermally regenerable resin, thus permitting an ion exchange treatment and a regeneration treatment to be carried out continuously and efficiently.
In the regeneration of a thermally regenerable resin, it is naturally desirable from the standpoint of economics to decrease as much as possible the amount of hot water used for the regeneration. In an up-flow type treating column, the raw liquid must be fed at a rate high enough to keep the thermally regenerable resin under sufficient upward force to ensure thorough ion exchange reaction. For this reason, the lower limit of flow rate of raw liquid is fixed by the specific gravity of the resin and other factors. If the flow rate is below this lower limit, it becomes necessary to take a counter measure, such as forming a supporting zone beneath the regeneration zone of thermally regenerable resin and feeding this supporting zone with sufficient water to increase the flow rate above such lower limit, the thereby to support the resin in the regeneration zone in a relatively fixed position. It is also necessary to interpose a heat displacement zone between the regeneration zone and the supporting zone to prevent possible loss of heat due to diffusion of heat below the regeneration zone.
To attain these objectives, experiments were conducted using a regeneration column having disposed therein a heat displacement zone immediately followed by a supporting zone. It was learned in the experiment that part of the water used as the supporting water would immediately undergo an ion exchange reaction (such as desalination). Having studied the effect of this supporting water in combination with the effect of the immediate ion exchange reaction on the water, the idea was conceived of having the ion exchange treatment and the regeneration treatment performed in a single column, disposing in this column a loading zone at a location corresponding to that of the supporting water function, and causing the flow of the raw liquid to play the part of supporting water. This conception is embodied in the present invention.